In the recent past there has been a new class of catalyst disclosed that produces new olefinic polymers and copolymers having many new and unique properties. In particular, these polymers offer strength properties based on their narrow molecular weight distribution. These polymers, however, are more difficult to process than their predecessors and thus methods of improving their processability are being sought. This invention relates to an improvement in the processing abilities of these and other narrow molecular weight distribution polymers.
Semi-crystalline polyolefins, such as polyethylene and polypropylene, are processed well above their melting temperatures. In the case of polyethylene, the processing temperature is usually in the range of 190.degree. to 260.degree. C. for extrusion and blow molding operations. For polypropylene in most of the cases, this temperature range is also suitable, although in case of specialty polypropylene it could be higher. The processing temperature and other parameters are obviously functions of the polymer grade and operation being sought. However, one thing is clearly emerging due to rapid advances in the processing and converting technology. It is desired that such materials be processed at the highest throughput rates possible with minimum energy consumption, that is, with the least possible head pressures during processing.
In order to accomplish this in the industry today, processing operations and screw designs are altered routinely for processing both conventional and specialty polyethylenes. The successful processing of non-conventional polymers such as narrow molecular weight distribution polyethylenes remains a challenge to the industry. In fact, even conventional so called linear polyethylenes (LLDPE) having a weight average molecular weight in the range of 35,000 to 200,000 and a molecular weight distribution of Mw over Mn less than equal to 5 cannot be processed at very high speeds using conventional polyethylene extruders. The latter is often desired and demanded due to obvious economic reasons. At high speeds in polyethylenes, as well as other conventional polymers such as polypropylene and polystyrene, flow instabilities occur during all sorts of processing operations including fiber spinning, extrusion, coating, film blowing and molding. Above a critical speed or, in other words, above critical stress and strain rates, the melt flow instabilities yield the extruded product which is highly distorted. In the extreme case, it can be chaotic. At times, even at relatively low speeds, the distortions of the extrudate of some grades of polyethylene can be bad enough to be readily detectable with the naked eye. Distortions, if limited to the surface, are called sharkskin. As the name implies, sharkskin consists of regular grooves and cracks perpendicular to the flow direction of the extrudate. Increase in the speed results in the increase of the severity of distortion, first appearing as a more wavy fracture then grossly helical distortions followed by at extremely high speeds, gross melt fracture and destruction of the extrudate. Such behavior obviously limits the throughput rates of polymers. This subject has been under study for nearly 30 years and continues to be investigated by industry and academia. The subject, for example, has been discussed in various polymer rheology books and chapters authored or edited by Eirich, Walters, Han, Keller (independent authors) and chapters in Encyclopedia of Polymer Science and Technology.
The phenomena of flow instability of high polymer melts at high shear rates is due to their inherent viscoelastic nature. The viscoelasticity of particular polymers is dictated by its molecular architecture and is the controlling factor in processing. Generally it is observed that polymers having broader molecular weight distributions, i.e., an Mw over Mn of greater than five can be processed at relatively high shear rates as compared to those having narrow molecular weight distributions. At any given shear rate and temperature, especially those employed in commercial processing operations, for high molecular weight polymers, the broad molecular weight distribution polymers have lower viscosities than corresponding narrow molecular weight distribution polymers.
Further, it is also an experimental fact that the drop to lower viscosity from the initial equilibrium viscosity, the so-called zero shear viscosity, .eta..sub.0, occurs at lower shear rates for broad molecular weight distribution than at narrow molecular weight distribution. Due to these two factors, at a given processing temperature and pressure, broad molecular weight polymers can be processed at corresponding higher rates than the narrow molecular weight counterparts. Conventional wisdom suggests that the lower molecular weight chains and broad molecular weight distribution polymers help in reducing not only the number of entanglements per unit volume, but also lower their rate of formation as well. Intuitively, thus, longer chains will have a higher probability of entangling than compositions with shorter chains. It is conjuncture that for this reason narrow high molecular distributed polymers have problems in processing at conventional and higher processing speeds.